The date that was yesterday until 3am

Fri, 11 Sep 2026 00:00:00 +0000

This line looks harmless:

const today = new Date().toISOString().slice(0, 10);

toISOString() gives the UTC date. I live in UTC+3, so from midnight until 03:00 every night that line names yesterday, and on the first of a month it names the previous month.

An app I maintain did it in three places: the today marker on a calendar, and a gate on the admin page that decides whether the month you’re editing is already in the past, twice. Nobody had noticed, because nobody uses a rostering app at 2am. Except the schedulers, who work nights. It surfaced in the review of the change that moved the app’s scripts out of its templates, as an old bug in code that change had only relocated.

Timezones on Wilhelm Maritz

The date that was yesterday until 3am